Code:
<script>
function changecolor(obj){
if (obj.checked) obj.parentNode.style.backgroundColor='green'
else obj.parentNode.style.backgroundColor='red'}
</script>
<form>
<div style="width:14px;height:14px;clip:rect(0 14 14 0);overflow:hidden;">
<span style="background-color: red;position:relative;top:-3px;left:-3px;">
<input onclick="changecolor(this)" type="checkbox" style="margin:0; padding:0; FILTER: progid:DXImageTransform.Microsoft.Alpha(style=0,opacity=50); -moz-opacity:0.5">
</span></div>
</form>
I like the image idea better